I'm having a similar issue.
When creating an event on my iphone, it syncs fine with google calendar and I can still edit it on my iphone. But once I update it on google calendar, the iphone calendar invite changes and I am no longer allowed to edit it - all I am allowed to do is "maybe", "decline", or "accept" the "invitation from <google calendar name>".
NOTE: On creation from iphone, the calendar was not an invitation from anyone but just an event I created.
More info here:
Have you set it up as a Gmail account or an Exchange account?
Also, have you gone to https://www.google.com/calendar/iphoneselect and checked all of the calendars to enable them to sync corectly?
If you have set it up as an Exchange acount, you'll need to go to m.google.com/sync instead